The conventional means of polishing or brightening aluminum uses phosphoric acid produced by the thermal process, known as the furnace process. This acid is manufactured in small quantities from elemental phosphorus, is more expensive and considerably more pure, and is usually reserved for processes requiring high purity phosphorus.
Wet process acid, on the other hand, is manufactured in large quantities directly from phosphate ores, is low cost and low purity, and is used primarily for fertilizers purified with a technical grade of phosphate salts.
Usually wet-process phosphoric acid is supersaturated with a group of sludge-forming components (Fe, Al, Ca, Mg, Cu, F, Na, K, Si, and SO.sub.4) that must be removed if purified phosphate salts are needed. However, the purification process is difficult and always results in the loss of phosphate values. Additionally, wet-process phosphoric acid is purified by solvent extraction, utilizing a number of different solvents including alcohols, such as amylbutylalcohol, or various ethers. These solvents tend to leave organic residues in the purified wet acid which react with the nitric acid in an aluminum polishing bath. For this reason, the aluminum cleaning industry customarily uses initially purer furnace grade phosphoric acid in its metal treatment processes due to the lower level of impurities.
The present invention teaches a new phosphoric acid bath and a new method for brightening aluminum. The primary acid is not the furnace acid customarily used, but wet-process phosphoric acid. This invention also teaches a method of producing an acid bath suitable for cleaning aluminum that is operable without the expensive extraction processes necessary to remove contaminants from wet-process acid. These contaminants, the ones listed above and in particular Mg, Fe, and Al, have heretofore reduced the effectiveness of conventional aluminum brightening baths. This invention also teaches a new aluminum brightening acid bath that does not require purifying the bath of all organic residues oxidizable in nitric acid. Other objects and advantages of this invention will become obvious to those skilled in the art from the following description.
In the typical process, an aluminum piece is immersed in a polishing bath for 0.5 to 4.0 minutes at a temperature of 102.degree. C. to 112.degree. C. The brightening bath contains approximately 80-50% phosphate acid and 3% nitric acid plus certain enhancers and defoaming agents.
The actual brightening of the metal surface is an electrochemical reaction--aluminum dissolves at the anodic sites and hydrogen evolves at the cathodic site. Microscopic galvanic cells cause an etching of the surface which, when properly controlled, produces a brightened surface. Chemical polishing occurs as minute protrusions on the surface of the metal are attacked, resulting in an increase in luminous reflectance. One method of controlling the polishing is the addition of heavy metal ions such as copper. These ions are cathodically reduced, forming a thin uniform precipitate on the surface of the aluminum.
Most brightening processes in the United States today use baths whose main constituent is phosphoric acid, a small amount of nitric acid, and a trace amount of copper. The present invention teaches a more complex bath, containing a variety of metal ions with 2.sup.+ and 3.sup.+ valences as well as specific amounts of sulfate and fluoride ions. The sulfate and fluoride ions inhibit the anodic attack while some of the metal ions are cathodically reduced to form a protective film on the aluminum surface.